1. Field of Invention
This application relates to printing machines having a bias transfer roller that transfers a toner image from an intermediate member, such as a belt, to a transfuse member, such as a belt, which then fuses the toner image to a recording medium, such as paper.
2. Description of Related Art
In a buffered belt transfuse system, conventional color toner separations are electrostatically transferred to a relatively thin intermediate belt in a plurality of first transfer nips. The full color image is then electrostatically transferred in a second transfer nip to a hot transfuse member (typically a transfuse belt). The intermediate belt heats up after passage through the second transfer nip. However, prior to the first transfer nip, the temperature of the intermediate belt is cooled and maintained at a stable temperature condition. In this manner, the imaging system is xe2x80x9cbufferedxe2x80x9d from the transfuse heat. The full color image on the transfuse belt is then rheologically transferred to paper in a third transfer nip.
Bias transfer rollers are conventionally used in the second transfer nip due to advantages caused by the addition of mechanical pressure at the second transfer nip. Additionally, the bias transfer rollers aid in reducing the intermediate belt heat thereby enabling shorter dwell time as compared to using corona transfer.
During standby, prior to engagement of the printing process, the bias transfer roller and the intermediate belt are disengaged from the hot transfuse belt in order to prevent reliability and life issues of the intermediate belt and bias transfer roller materials.
Accordingly, at the start of the printing process, the bias transfer roller can take a substantially long time to cycle up to its higher steady state temperature after nip engagement. Initially, the bias transfer roller is engaged in nip forming contact with the hot transfuse belt. This engagement causes the bias transfer roller to heat up. At an extreme start up condition, the bias transfer roller temperature is initially at room temperature and eventually cycles up to a much higher steady state temperature condition. The steady state condition depends on parameters such as the initial intermediate belt temperature, the transfuse belt temperature, the second transfer nip contact dwell time, etc.
With typical 6 mm. thick bias transfer roller rubber layers, the bias transfer roller can take a substantial duration of time to cycle up to the higher steady state temperature after. For example, typically the bias transfer roller will take more than about 20 minutes to cycle up to a steady state value of around 70xc2x0 C. under typical nip dwell conditions where the initial intermediate belt temperature is maintained at about room temperature and the transfuse belt is maintained at about 120xc2x0 C. The bias transfer roller temperature swings can even be larger at higher transfuse belt temperatures or longer bias transfer roller nip dwell times. To the disadvantage of conventional transfuse systems, after nip engagement with the transfuse belt, the bias transfer roller moves through a substantially wide temperature swing thereby requiring a substantially long cycle up period.
Bias transfer roller transfer prefers an optimum range of restivities in order to achieve wide operating transfer latitude. Ideally, the bias transfer roller resistivity is maintained over a very narrow range of optimum values in order to achieve stable, optimum transfer performance. Conventional systems can sometimes accept around a 10xc3x97 variation in resistivity by using constant bias transfer roller current or other power supply control approaches that tend to compensate somewhat for the effects of changing bias transfer roller resistivity. However, usually this requires some transfer latitude help via optimized toner design for transfer and it usually also requires some tradeoff compromise in performance at the extremes of the bias transfer roller resistivity variations. More ideally, the resistivity variation is less than 3xc3x97 in a system for very robust performance. Unfortunately, the resistivity of conventionally available bias transfer roller materials is significantly dependent on the bias transfer roller temperature. For example, the resistivity of many ionic filled bias transfer rollers can change by more than three orders of magnitude when the temperature changes between about 25xc2x0 C. and 120xc2x0 C. The bias transfer roller temperature swings that occur in a transfuse system can thus cause significant bias transfer roller latitude issues for transfuse systems.
Additional bias transfer roller problems caused by exposure to elevated temperatures in the conventional transfuse system exist. For example, some bias transfer roller materials can have increased mechanical degradation problems due to the elevated temperature. Also, long term exposure to the combination of elevated temperature and high transfer electrostatic field cause significant drift in the electrical and mechanical properties of some readily available bias transfer roller materials. For such materials, it is advantageous not to expose the bias transfer roller to elevated temperatures. Since bias transfer roller material development is difficult and generally involves long manufacturing development and qualification cycles in order to meet all of the mechanical, electrical, and life requirements needed for bias transfer rollers, an alternate solution to material processing is desirable.
Furthermore, the electrical properties of various bias transfer roller materials have the tendency to drift with use, even at room temperature. Accordingly, bias transfer roller aging and life issues are evident. For optimum and robust bias transfer roller performance, it is desirable to implement a system that compensates for long term drift in the electrical properties of the bias transfer roller.
U.S. Pat. No. 6,088,565 to Jia et al., the entire disclosure of which is incorporated herein by reference, discloses a conventional transfuse system in which plural toner image forming stations form toner images on an intermediate transfer member, and then the composite toner image is transferred to a transfuse member at a second transfer nip. Jia et al. does not disclose controlling, or recognize the need to control, the bias transfer roller temperature.
U.S. Pat. No. 5,321,476 to Gross, the entire disclosure of which is incorporated herein by reference, discloses a bias transfer roller including an internal heating element. The Gross system is not a transfuse system; rather, Gross uses a bias transfer roller to directly transfer a toner image to a sheet of paper. Since the toner image is fused to the paper at a separate location, the bias transfer roller is not subjected to heat from the fuser. In addition, Gross does not disclose controlling the bias transfer roller temperature by using an external temperature control device.
This invention has been made in view of the above circumstances. The present invention addresses the long-standing problems discussed above by controlling the temperature of the bias transfer roller in a transfuse system during standby and/or after nip engagement in order to provide and maintain optimum transfuse system bias transfer roller resistivity ranges in a transfuse system.
One aspect of this invention is to control the temperature of the transfuse system bias transfer roller by cooling the bias transfer roller to avoid excessive bias transfer roller heating and to maintain the bias transfer roller within an optimum temperature range.
Another aspect of this invention is to provide temperature control to the transfuse system bias transfer roller by heating the bias transfer roller, e.g., during standby, thereby avoiding long term cycle up changes after nip engagement.
In accordance with another aspect of this invention, a control system is provided that compensates for possible long term drift of the bias transfer roller electrical properties by periodically updating the temperature control setting of the bias transfer roller. The control system monitors the bias transfer roller voltage needed for a given bias transfer roller current and chooses an updated bias transfer roller temperature control setpoint.